Behind the scenes: What goes on in the flight deck during a diversion?
From takeoff until touch down, most flights run pretty smooth. As airline pilots, we do this hundreds of times a year. However, every so often, something happens that means we are unable to land at the planned destination. Be it bad weather, technical issues or problems in the passenger cabin, once in a while we have to change our plans and divert to another airport. Broadly speaking, most people are not a fan of change. We like to have a plan and stick to it. Yet, as airline pilots, we have to be constantly open to change and have the ability to mix up our plans at a moment's notice. This is how we handle an in-flight diversion.
Why do flights get diverted?
Whilst an in-flight diversion isn't a regular occurrence, flying an airliner is an ever-changing environment. We are always aware that we may need to change our plans at any time, no matter what the stage of flight. Most diversions occur for three main reasons: bad weather at the destination airport, technical problems with the aircraft or issues with passengers.
Before every flight, we check the weather forecast for our estimated time of arrival at the destination airport. On short-haul flights, this is normally only a couple of hours away. However, on long-haul flights, this arrival time could be almost 24 hours later. As a result, the accuracy of weather forecasts is extremely important. When approaching the destination airport, to ensure the safety of the aircraft and its occupants, there are is a minimum criteria of weather in which aircraft are allowed to make an approach to land. Broadly speaking, this is determined by the visibility on the ground and, to a certain extent, how low the cloud is. These limits vary from airport to airport, runway to runway and aircraft type to aircraft type.
If the reported weather is worse than the published minima, the pilots are legally not allowed to make an approach to land. Sometimes, the weather may be reported as good, but as we approach the runway, if we do not see the visual clues out of the window that we require to land, we must perform a go around and head back up into the air. If there is no improvement to the weather, it may be necessary to divert to another airport with better weather conditions.
Modern airliners are hugely complex machines with millions of different parts. Like with any machine, systems do go wrong. With that in mind, aircraft designers have anticipated this and built in backup systems should the primary system fail. In some cases, there are even back ups for the backup systems. This not only enhances flight safety, but it also reduces the impact on punctuality when there are technical issues. That said, every so often, technical problems occur that make it necessary for the pilots to divert the aircraft. This is very rarely a safety issue, but more of an operational issue. For example, an aircraft taking off from London to fly to New York develops a problem with one of its systems when over Ireland. While this isn't a safety issue and the pilots could safely continue to New York, the engineers in New York may not have the spare parts to fix the problem when the aircraft lands. It could then potentially be stuck there for days on end, causing disruption across the airline's entire network. As a result, the airline's operations department may deem it better for the aircraft to return to London to be fixed at the main maintenance facility, rather than continue to New York.
If you take 400 random individuals off the street and put them in a room, you're going to get a real mix of people. Some old, some young. Some healthy, some fit. Some with mental health problems, others with physical disabilities. No matter who we are, we're are all fighting our own battles, battles which are quite often invisible to those around us. If you then take that mix of 400 people and load them into a pressurized metal tube for 20 hours, some crammed in tight economy seats, often with alcohol readily available, it's easy to see how problems may arise.
On most long-haul flights, at least one passenger will suffer from some sort of medical issue. Long journeys combined with age and ill health are a perfect recipe for an individual to feel a little unwell. Fortunately, the cabin crew are trained to deal with a number of medical conditions and are able to deal with most of these issues. Should a passenger become seriously unwell, it may be necessary to divert the aircraft so that they can receive better medical care from doctors and paramedics on the ground.
Related reading: What happens if you get ill on a flight?
Human behavior can also be a cause of problems in the passenger cabin. As an industry, we are seeing more and more incidents of disruptive passengers. As pilots, we have a responsibility to ensure not only the safety, but also the customer experience of all passengers on board the aircraft. If the behavior of an individual is deemed a threat to the safety of the aircraft, or those in it, the decision may be made to divert the aircraft and offload that person.
An aircraft's time in the air is always limited. There's no option to just pull over and think things through. As a result, the decision on how much fuel to take at the start of the flight is a complex and critical one. Careful study of the weather forecast will alert us to any potential problems at the destination. If the weather is bad enough to stop aircraft from making approaches, or the airfield needs time to clear snow from the runways, aircraft will have to enter holding patterns and wait. If these conditions are expected, pilots will most likely take more fuel than normal. In addition to this, all flights must carry separate fuel that enables them to fly from the planned destination airport to another airport where they can land safely. Depending on geographic location and weather, this may be an airport a few miles away. Or, in the case of islands like the Seychelles, the diversion airport could be several hours away. This planned diversion airport will be selected on the basis that the weather forecast will be good enough to land.
Related reading: How do pilots know how much fuel to take on a flight?
Deciding to divert
Like with many decisions in life, there's not always a simple answer. Depending on the problem and all the variables that come with it, there may be a number of viable options to take. In-flight, quite often the most important factor is time. How much time do we have to make this decision? If we have gone around after making an approach in bad weather but have enough fuel to hold for an hour, the time pressure to make the decision to divert isn't particularly high. If, however, a passenger has suffered a serious heart attack, the time taken to make the decision could be the difference between life and death.
Technical problems can vary, depending on the severity of the issue. If there's a problem with the toilets, there's plenty of time to have a conversation with the operations department as to what they feel would be best for the network operation. However, if we have to shut an engine down due to a surge or overheat, there's no time to have these chats. This is why, at any stage of flight, a good crew will know exactly where their closest diversion airfield is where they can safely land, given the current weather and aircraft conditions.
In the flight deck
Once the decision has been made to divert, the workload in the flight deck increases dramatically. There are a number of tasks that must be completed before we are able to land at the diversion airport, so dividing the jobs between both pilots (or three pilots on a long-range flight) is key. The time available is the driving force in prioritizing those tasks. If you've read any of my previous stories, you'll probably be familiar with the "Fly, Navigate, Communicate" philosophy. If not, the basic premise is that before engaging in any other tasks, make sure one pilot's sole focus is on the flight path of the aircraft. Next, they need to make sure that the aircraft is traveling exactly where they want it to be. Finally, the other pilot can then communicate with the relevant people to arrange the diversion.
The first task is to get the aircraft pointing in the right direction. In most cases, permission will be required from ATC to deviate from our route. However, with technical issues, we may need to get the aircraft moving away from its current flight path before we are able to talk to ATC. In the case of an engine shut down over the Atlantic, this will be done within a few seconds.
Related reading: How pilots handle an in-flight engine shut down
In the event of a medical or weather-related diversion, there will be time to relay our intentions to ATC before changing course.
With weather-related diversions, we know in advance how much fuel we will need to fly to the diversion airport. With this in mind, we’ll be able to calculate an exact time at which we will have to give up waiting to land at the planned destination and head to the diversion airport. By letting ATC know this well in advance, it won’t come as a surprise to them when we do ask to divert. With medical diversions, whilst it may be important to get the sick passenger on the ground as soon as possible, the safety of everyone else on board is still the most important factor. As a result, coordinating a new route with ATC is imperative before starting the diversion.
With the diversion approved by ATC, we can start to reprogram the Flight Management Computer (FMC) to navigate the aircraft toward the diversion airport. This requires one pilot to change the destination and then enter the route as prescribed by ATC. Once we know where we’re going laterally, we also need to think about what we’re doing vertically. The diversion may take us across a mountain range. If so, what’s the lowest altitude we can safely descent to? A new Minimum Enroute Altitude (MEA) must be confirmed by checking the onboard charts. The next task is to obtain the latest airfield information from the diversion airport. This will provide the runway and approach in use and also the weather conditions. Once these are known, further changes are made to the FMC to ensure that the aircraft is set up to fly the desired approach type. The airfield charts must then be selected on both pilots' iPads to prepare for an arrival brief at the new airport.
The final part of the process is nice to do but not necessarily required. As mentioned above, the time available is what prioritizes the tasks. This is why the communicate part comes last. As we will already have spoken to ATC, this part will involve letting all the relevant stakeholders aware of our diversion. First up is normally the cabin crew. If it’s a medical diversion, they will probably already be aware that the plan is to divert. However, with a technical or weather diversion, they most probably will not be aware. With the crew aware of the plan, we are then able to speak to the passengers over the PA system. The crew will then be able to answer any questions they may have. Finally, if there is time, we will try to let the airline’s operations department know that we are diverting. This will give them the opportunity to contact the ground handling agents and give them time to prepare for our arrival. However, if time is short, this step will be omitted.
The decision to divert an aircraft is never taken lightly. There are always large costs involved with a diversion — both operationally and financially. That said, safety is your pilot's number one priority and nothing will come between them and the safe outcome of the flight. Once on the ground at the diversion airport, things often take time to progress. The airline's operations department may have no idea that the aircraft has diverted until the pilots call from on the ground. The safe flying of the aircraft always takes priority over communicating the plan with other people. This is why things often seem to be chaotic and disjointed once on the ground. The crew and the airline need time to come up with a plan. Whatever the reason for a diversion, rest assured that it was done purely for the safety for those on board the aircraft.